A typical fuel cell system consists of a number of fuel cell units arranged in a stack; each fuel cell unit includes anode and cathode which are provided on either side of electrolyte. Hydrogen, which functions as a fuel, is supplied to the anodes while air, which functions as an oxidizing gas, is supplied to the cathodes. Catalytic reaction within fuel cell generates electric power. The difference between solid oxide fuel cells (SOFC) and proton exchange fuel cells lies in construction materials and operating temperatures that determine the type of catalytic reaction.
But for both SOFC and PEM FC effective hydrogen supply to anode active centers requires optimal water balance and removal of by-product inert gases. Hydrogen-containing gases supplied from reformer has high content of inert gases-impurities that block access of working reagent to active centers. This effect is known as a “blanket”. To destroy such “blocking blanket” a gas recirculation with help of fans, diaphragm pumps or ejectors is used. During PEM FC operation and, especially, re-activation nitrogen is accumulated at fuel cell stack's anode chambers (up to 60%), including supply of pure hydrogen to anodes and air to cathodes. It should be noted that in order to effectively remove impurities at low loads the anode gas recirculation above “stoichiometric” is required because the anodes should be ready for maximal loads at any time. In existing PEM FC designs electromechanical fans or blowers are used. They are provided increased flow speed of recirculating anode gas (in 2.5 . . . 3.5 times) above that required at maximal load. It is also known that grease is not allowed for internal surfaces of gas reagents supply devices. It should be also noted that in hydrogen environment most materials absorb the hydrogen. Hydrogen diffuses into metals, plastics and rubbers; the diffusion is increased significant at high temperatures. As a result the metals or plastics embrittlement and construction destruction are observed, beginning with support bearings that operate at high revolutions, about (15 . . . 17)×103 rpm. Under such circumstances electromechanical grease-free fans show very low service-life, less than 700 hours.
Therefore gas jet ejector as an anode gas circulator arouses interest because it allows providing gas recirculation with directing high pressure working gas through an ejector nozzle. A typical ejector or a Venturi tube can ideally serve as a gas circulator at high pressure and slightly or non-varying gas flow speed downstream nozzle. In case of mobile fuel cell systems with loads and working gas flow speeds varying irregularly from a “near zero” level to maximum or peak level, said parameters vary by 100 and 1000 times accordingly. Injection and gas dynamic pressure characteristics also vary in wide range. Reaching optimal mass exchange in fuel cells is the largest problem at “near zero” and minimal loads no greater than 35% from maximal load.
An attempt to adjust gas flow speed with changing the nozzle cross-section using a central rod does not solve the problem because working gas flow speed through the ejector nozzle sharply decreases and at the same time the basic geometrical characteristic of the ejector increases resulting in decrease of ejection factor (anode gas recirculation). The basic geometrical characteristic of the ejector is a ratio of ejector mixing chamber area to an area of working gas jet ejecting through the nozzle. This characteristic has a main effect on the output performance of the ejector: ejection factor and gas dynamic pressure.
For example in U.S. Pat. No. 6,868,340 nozzle output and mixing passage areas are adjusted simultaneously using a predetermined profiled needle. The needle moves along the central axis of nozzle and mixing passage. The area of an opening around the needle in the opening of the nozzle is changed synchronously with the area around another portion of the needle in the inlet part of the mixing passage or diffuser. The needle is movable along the central axis by diaphragms wherein the end of needle is inserted. The pressures from FCS anodes and cathodes are applied to the diaphragms. The drawback of such approach is significant decrease of the flow speed through the nozzle and mixing passage. This results in decrease of ejector output characteristics by 30 . . . 40% and full inefficiency at minimal loads with near zero flows.
It is known fact that in mobile applications as well as other applications they provide a power reserve in case of “peak” power loads for a short period of time. Most periods of time (85%) mobile power generating systems operate at power loads less than 35% of maximal design power. Maximal or “peak” powers are used within 2% of total operational period. Other 10 . . . 15% of time slightly exceed 35% of maximal power. Thereby it is worth to optimize the anode gas supply flows in order to intensify anode gas recirculation at variable loads. In this connection multi-stage ejectors are used for gas supply when two or more ejectors operate separately or simultaneously. Such approach is used in US Application Publication No. 2005/0064255 that includes a high-flow ejector and low-flow ejector with common mixing chamber. Having two ejectors leads to space requirement concern and a transition point when gas flow changes from low-flow to high-flow ejector, experiences a drop in recirculation flow.
The present invention addresses these and other needs and provides further related advantages.